LEDs are solid state lighting devices that convert electric energy into light. LEDs include both semiconductor-based LEDs and organic LEDs. Semiconductor-based LEDs typically include a plurality of semiconductor layers that are epitaxially grown on a semiconductor or non-semiconductor substrate such as, for example, sapphire, silicon, silicon carbide, gallium nitride or gallium arsenide substrates. One or more semiconductor p-n junctions are formed in these epitaxial layers. When a sufficient voltage is applied across the p-n junction, electrons in the n-type semiconductor layers and holes in the p-type semiconductor layers flow toward the p-n junction. The epitaxial structure may include cladding layers, quantum wells or the like that are designed to trap some of the electrons and holes in the vicinity of the p-n junction. When an electron and a hole collide they recombine and a photon of light is emitted, which is how LEDs generate light. The wavelength distribution of the light generated by an LED generally depends on the semiconductor materials used and the structure of the thin epitaxial layers that make up the “active region” of the device (i.e., the area where the electrons and holes recombine).
The peak wavelength of an LED refers to the wavelength where the radiometric emission spectrum of the LED reaches its maximum value as detected by a photo-detector. The radiometric emission spectrum, which is also referred to as the “spectral power distribution” of the LED, is a plot of the radiant flux for the light emitted by the LED as a function of wavelength. The radiant flux of the LED (which is also referred to as “radiant power”) is a measure of the intensity of the light emitted by the LED in Watts (or an equivalent unit of measure). An LED typically has a radiometric emission spectrum that has a narrow wavelength distribution that is tightly centered about the peak wavelength of the LED. For example, the radiometric emission spectrum of a typical LED may have a width of, for example, about 10-30 nm, where the width is measured at half the maximum illumination (referred to as the full width half maximum or “FWHM” width).
LEDs may also be identified by their “dominant” wavelength, which is the wavelength where the radiometric emission spectrum of the LED, as perceived by the human eye, reaches its maximum value. The dominant wavelength thus differs from the peak wavelength in that the dominant wavelength takes into account the sensitivity of the human eye to different wavelengths of light.
Most visible light sources emit light at many different wavelengths. The apparent color of visible light can be illustrated with reference to a two-dimensional chromaticity diagram, such as the 1931 CIE Chromaticity Diagram illustrated in FIG. 1. Chromaticity diagrams provide a useful reference for defining colors as weighted sums of different colors.
As shown in FIG. 1, colors on a 1931 CIE Chromaticity Diagram are defined by x and y “chromaticity” coordinates. Each (x, y) value in the 1931 CIE Chromaticity Diagram of FIG. 1 is a distinct “color point.” As shown in FIG. 1, the color points within the visible light spectrum fall within a generally U-shaped area. Colors on or near the outside of the area are “saturated” colors composed of light having a single wavelength (monochromatic light), or light having a very small wavelength distribution. The numerical values specified about the periphery of the U-shaped curve of FIG. 1 show the wavelength of monochromatic light that corresponds to various points on the curve. Colors in the interior of the U-shaped area of FIG. 1 are “unsaturated” colors that are composed of a mixture of different wavelengths of light. White light, which can be a mixture of many different wavelengths, is generally found near the middle of the diagram, in the region labeled 10 in FIG. 1. There are many different hues of light that may be considered “white” or near white, as evidenced by the size of the region 10.
Light that generally appears green or includes a substantial green component is plotted in the regions that are above the white region 10, while light below the white region 10 generally appears pink, purple or magenta. Light that generally appears red falls in the lower right hand side of the U-shaped region of FIG. 1, while light that generally appears blue falls to the left of the white region 10 of FIG. 1.
A binary combination of light from two different light sources will appear to have a different color than either of the two constituent colors, where the color of the combined light will depend on the wavelengths and relative intensities of the two light sources. For example, light emitted by a combination of a blue source and a red source may appear purple or magenta to an observer.
As most LEDs are saturated light sources that appear to emit light having a single color, LED-based light emitting devices that produce white light have been introduced in which light emitted by an LED is passed through one or more luminescent materials (such as phosphor particles) that convert some of the light emitted by the LED to light of other colors. The combination of the light emitted by the single-color LED that passes through the luminescent material along with the light of different colors that is emitted by the luminescent materials may produce white or near-white light. For example, a single blue-emitting LED (e.g., made of indium gallium nitride and/or gallium nitride) may be used in combination with a yellow phosphor such as for example, cerium-doped yttrium aluminum garnet (Y3Al5O12:Ce), that “down-converts” the wavelength of some of the blue light emitted by the LED, changing its color to yellow. In a blue LED/yellow phosphor lamp, the blue LED produces an emission with a dominant wavelength of, for example, about 455-470 nanometers, and the phosphor produces yellow fluorescence with a peak wavelength of, for example, about 550 nanometers in response to the blue emission. Some of the blue light passes through the phosphor (and/or between the phosphor particles) without being down-converted, while a substantial portion of the light is absorbed by the phosphor, which becomes excited and emits light across a broad spectrum that has a peak wavelength in the yellow color range (i.e., the blue light is down-converted to yellow light). The combination of blue light and yellow light may appear white to an observer.
LEDs are used in a host of applications including, for example, backlighting for liquid crystal displays, indicator lights, automotive headlights, flashlights and for general illumination.